Multilayer optical films, i.e., films that provide desirable transmission and/or reflection properties at least partially by an arrangement of microlayers of differing refractive index, are known. It has long been known to make such multilayer optical films by depositing a sequence of inorganic materials in optically thin layers (“microlayers”) on a substrate in a vacuum chamber. Typically, the substrate is a relatively thick piece of glass, limited in size due to constraints on the vacuum chamber volume and/or the degree of uniformity possible by the deposition process.
More recently, multilayer optical films have been demonstrated by coextrusion of alternating polymer layers. See, e.g., U.S. Pat. Nos. 3,610,724 (Rogers), 4,446,305 (Rogers et al.), 4,540,623 (Im et al.), 5,448,404 (Schrenk et al.), and 5,882,774 (Jonza et al.), the disclosures of which are incorporated herein by reference in their entireties. In these polymeric multilayer optical films, polymer materials are used predominantly or exclusively in the makeup of the individual layers. Such films are compatible with high volume manufacturing processes, and can be made in large sheets and roll goods.
Many product applications, however, require relatively small and numerous pieces of film. Filters for individual photodiode detectors is one such application. Windows, reflectors, and/or filters for fiber optic devices and other small-scale photonics devices are additional applications. For these applications, small pieces of multilayer optical film can be obtained from a larger sheet of such film by subdividing the sheet by mechanical means, such as by cutting the sheet with a shearing device (e.g., a scissors), or slitting the sheet with a blade. However, the forces exerted on the film by the cutting mechanism can produce layer delamination in a region along the cut line or edge of the film. This is particularly true for many polymeric multilayer optical films. The delamination region is often discernable by a discoloration relative to intact areas of the film. Because the multilayer optical film relies on intimate contact of the individual layers to produce the desired reflection/transmission characteristics, the delamination region fails to provide those desired characteristics.
In some product applications, the delamination may not be problematic or even noticeable. In others—particularly where it is important for substantially the entire piece of film from edge to edge to exhibit the desired reflection or transmission characteristics, or where the film can be subjected to mechanical stresses and/or wide temperature variations that could cause the delamination to propagate in the film over time—the delamination can be highly detrimental. Also in some cases some amount of delamination may be difficult to avoid due to a need to mechanically cut or sever at least a portion of a periphery of a piece of multilayer optical film.
There exists, therefore, a need for controlling delamination in multilayer optical films. Preferably, the approach would be compatible with automated and/or continuous manufacturing processes.